The use of inorganic fibers and whiskers to reinforce such matrices as glasses, glass-ceramics, sintered ceramics, organic plastics, and various metals is well known. Fibers and whiskers have been commonly distinguished upon the basis of their microstructure. Thus, whiskers have generally been defined as elongated, single-crystal fibers. Hence, whiskers have typically been described as having a thickness less than about 100 microns with a length-to-diameter ratio of at least 10. In contrast, fibers are customarily considered to be multicrystalline or amorphous.
Because of their intrinsic shape, high modulus of elasticity, and high tensile strength, inorganic whiskers of different compositions can be implanted in various matrices to produce composite products exhibiting superior strength-to-weight and stiffness-to-weight characteristics. To illustrate, whiskers prepared from very stiff, low density covalent compounds such as carbides, nitrides, and oxides can demonstrate elastic moduli greater than most metals and are often much stronger than steel, when viewed in proportion to their weight.
Much study has been reported in the scientific literature to comprehend the fundamental mechanism underlying the strengthening improvement to composite articles imparted through the presence of fibers therein. As a result of that study, it has been generally agreed that the mechanism operating involves a load transfer by the matrix to the fibers through shear; that load transfer taking place near the ends of the fibers within a distance of a few fiber diameters.
A like strengthening mechanism has been postulated to be operating in whisker-containing composites, but the extent of the load transferred by the matrix to the whiskers is a function of the length and aspect ratio of the whisker. As a consequence, a whisker may not be loaded to its breaking stress such that full advantage cannot be taken of its reinforcing capability. In addition to the factors of length and aspect ratio of a whisker, orientation of a whisker with respect to applied stress and stress concentrations at the ends of the whiskers results in lower strength improvement than is possible with fibers. Therefore, whisker reinforced composites will normally exhibit lower mechanical properties than unidirectionally-oriented, continuous fiber reinforced composites prepared from like components (when properties are determined along the fiber axis). Whisker-containing composites possess one substantive advantage over the continuous fiber-containing composites, however, in that they are macroscopically isotropic.
U.S. Pat. No. 4,464,192 discloses the preparation of reinforced composite articles consisting of whiskers or chopped fibers embedded in a glass matrix. The patent describes in some detail the production, through injection molding, of composite articles consisting of chopped fibers (&lt;0.75" in length with an average diameter of .about.5-50 microns) of alumina, graphite, silicon carbide, and/or silicon nitride dispersed within a matrix of a high silica glass, or a borosilicate glass, or an aluminosilicate glass. As a specific example of a high silica glass, the patent cites Corning 7930, a glass containing about 96% SiO.sub.2 marketed by Corning Glass Works, Corning, N.Y. under the trademark VYCOR; as a specific example of a borosilicate glass, the patent cites Corning 7740, marketed by Corning Glass Works under the trademark PYREX; and as a specific example of an aluminosilicate glass, the patent cites Corning 1723, an alkaline earth aluminosilicate glass marketed by Corning Glass Works.
The patent observes that a mixture of glass powder and chopped fibers or whiskers may be injected directly into a mold. However, the preferred forming method comprises the following steps:
(a) admixing a polymeric binder, glass powder, chopped fibers, and a carrier liquid; PA0 (b) drying the mixture; PA0 (c) cold pressing the mixture to less than its original volume; PA0 (d) heating the pressed mixture to eliminate the carrier liquid and binder; PA0 (e) hot pressing the mixture to form a billet of a desired density; PA0 (f) placing the billet in an injection molding apparatus; PA0 (g) heating the billet to the softening point of the glass; and PA0 (h) injecting the softened billet into a preheated mold having the desired internal configuration. PA0 (a) a tow of fibers from a spool was passed through a slurry of powdered glass to impregnate the tow; PA0 (b) the impregnated fibers were re-wound onto a larger spool in such a manner to form a tape; PA0 (c) the tape was dried, removed from the receiving spool, and cut into strips to conform to the dimensions of the article desired; PA0 (d) the strips were laid up in a mold; and then PA0 (e) hot pressed to form a composite body. PA0 (a) forming a slurry of the whiskers utilizing a polar carrier liquid; PA0 (b) stirring the slurry; and PA0 (c) ballmilling the slurry. PA0 (a) SiC whiskers are blended into a glass powder as described above; PA0 (b) that mixture combined with continuous (non-chopped) ceramic fibers to form oriented prepregs; PA0 (c) the prepregs are stacked in a desired sequence of ply orientations; and PA0 (d) the stacked prepregs are fired at an elevated temperature to consolidate the stack into an integral body. (Customarily, the stack will be hot pressed or isostatically pressed to insure the production of void-free bodies.)
The preferred carrier liquid is water and the preferred binder is a polymeric wax. The fibers comprise about 15-50% by volume of the final composite body.
U.S. Pat. No. 4,263,367 discloses the production of reinforced composite articles consisting of discontinuous graphite fibers (average length .about.0.75") embedded in a glass matrix. The single specific glass referred to in the patent was Corning 7740. In the preferred embodiment of the method to prepare such bodies, plies of graphite paper were either dipped in a slurry of powdered glass and then stacked, or simply stacked with layers of powdered glass between each ply of paper, and the as-formed articles hot pressed to form composite articles. The fibers comprised about 20-50% by volume of the articles.
U.S. Pat. No. 4,314,852 discloses the fabrication of reinforced composite articles consisting of continuous SiC fibers embedded in a glass matrix, the glass being selected from the group of high silica glass, borosilicate glass, and aluminosilicate glass. In like manner to U.S. Pat. No. 4,464,192 above, Corning 7930 comprised the high silica glass, Corning 7740 comprised the borosilicate glass and Corning 1723 comprised the aluminosilicate glass.
The method provided for producing such articles involved the following steps:
The fiber content comprised about 30-70% by volume of the final product.